<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <script>
        var num = 0
        // for (i = 1; i<= 100; i++) {
        //     num = num + i   
        // }
        // alert(num);

        // 100内偶数合
        for (i = 1; i <= 100; i++) {
            if (i % 2 == 0) {
                num = num + i
            }
        }
        alert(num)
    </script>
</body>

</html>